This invention is directed to electrolytically coating of one side only of a horizontally moving flat rolled strip or sheet. More particularly, though not limiting, this invention is directed to a method of and apparatus for electrogalvanizing a continuous ferrous strip.
The forerunner of today's high speed electrolytic plating lines dates back to the early 40's when a Government directive urged installation of electrotinning units to conserve tin. From this evolved the Halogen Tin Process. For a discussion of said process, see Plating and Surface Finishing, February, 1976, pages 44-49, an article entitled "Development of the Halogen Tin Process for High Speed Plating of Wide Steel Strip," by E. J. Smithh et al. Additionally there are a number of patents which describe improvements in or modifications to the Halogen process and apparatus for practicing same, note particularly U.S. Pat. Nos. 3,691,049--Eppensteiner et al, 3,645,876--Wilson, 3,264,198--Wells, 2,758,075--Swalheim and 2,569,577--Reading.
A characteristic feature of a high-speed Halogen process, as exemplified by the above noted prior art, is that during high-speed operation, strip drag causes plating solution to pile up and flood the strip at the cell exit end. The cell exit rolls serve as plating solution dams. The dammed plating solution spills out board from the strip pass line and flow, by gravity, along the cell side walls counter to the travel of the strip. Eventually, the plating solution reaches the cell entry end where it spills over a weir.
At high strip line speeds, which may be as high as 610 m/min (2000 ft/min)--E. J. Smith et al article, supra, plating solution overflow begins about midway through the cell and progressively increases until plating begins on the upper surface. That is, edge plating begins when there is sufficient plating solution present to carry current. It will be understood that there is an inherent resistance to plating until the potential across the sheet is overcome. For a high speed plating operation it is difficult to quantify how much plating solution is necessary to overcome the inherent resistance to plate. However, experience has shown that the flooding conditions of the conventional Halogen line are more than sufficient to cause at least edge plating. Thus, the practices represented by the above patents, though several purport to coat only one side (bottom) of a moving strip, suffer the drawback of not fully preventing some coating on the upper surface. Such is obviously not a problem where ultimately both sides of a strip are to be coated. However, where one side of the strip is to remain clean, such as is essentially mandatory for automotive applications, costly procedures must be followed to clean the upper or partially coated surface. The present invention accomplishes the preceding by minimizing contact between the plating solution (electrolyte) and the upper surface. In practice the moving strip serves as a stop or solution deflector and causes said solution to flow along and toward the strip edges to extend outward in a flat flow pattern. This will be described in greater detail hereinafter.